Really, quite literally, some pretty skilled Microsoft employee or contractor reverse engineered our friend EQNEDT32.EXE, located the flawed code, and corrected it by manually overwriting existing instructions with better ones (making sure to only use the space previously occupied by original instructions).

This... This is one hell of a story. The unanswered question is why, exactly, Microsoft felt the need to do this - do they no longer have access to the source code? Has it simply become impossible to set up the correct build environment?